Patent Application
20110296866
The present invention relates to a method of capturing carbon dioxide in a fluid comprising at least one compound more volatile than carbon dioxide CO2, for example oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.
The invention can be notably applied to units producing electricity and/or steam from carbon fuels such as coal, hydrocarbons (natural gas, fuel oil, petrochemical residue, etc), household waste, biomass but can also be applied to gases from refineries, chemical plants, steel-making plants or cement works. It could also be applied to the flue gases from boilers used to heat buildings or even to the exhaust gases from transport vehicles, and more generally to any industrial process that generates CO2-containing flue gases.
Carbon dioxide is a greenhouse gas. For environmental and/or economic reasons, it is becoming increasingly desirable to reduce or even eliminate discharges of CO2 into the atmosphere by capturing it and then, for example, storing it in appropriate geological layers or by realizing it as an asset in its own right.
A certain number of techniques for capturing carbon dioxide, for example methods based on scrubbing the fluids with solutions of compounds that separate the CO2 by chemical reaction, for example scrubbing using MEA, are known. These methods typically have the following disadvantages:
In the field of cryo-condensation, that is to say of cooling until solid CO2 appears, mention may be made of document FR-A-2820052 which discloses a method allowing CO2 to be extracted by anti-sublimation, that is to say by solidification from a gas without passing via the liquid state. The cold required is provided by means of fractionated distillation of refrigerating fluids. The fluid containing CO2 and at least one compound more volatile than CO2 is separated by alternate condensation in two vessels. While the CO2 is condensing in one vessel, the CO2 is subliminated in the other, raising the pressure to the triple point for CO2 before the liquid CO2 thus created is extracted. This method has the following disadvantages:
Document US 2005/072186 describes a method for purifying natural gas containing CO2. The mixture is cooled and solid CO2 is formed within the liquefied natural gas. A “soup” of solid CO2 and LNG is extracted via an endless screw, then heated up to liquefy the CO2. One disadvantage of this method is that a certain amount of liquefied natural gas is extracted at the same time as the solid CO2, and this then has to be recirculated. Further, the CO2 forms blocks of solid which are not easy to extract.
It is one object of the present invention to provide an improved method of capturing carbon dioxide from a fluid containing CO2 and at least one compound more volatile than the latter.
The invention relates first of all to a method for producing at least one CO2-lean gas and one or more CO2-rich fluids from a process fluid containing CO2 and at least one compound more volatile than CO2, said method implementing:
a) a cooling in at least one vessel of at least part of said process fluid so as to obtain at least one solid containing predominantly CO2 by cryo-condensation of part of the process fluid and at least one residual gas containing said CO2-lean gas;
b) an extraction from said vessel of at least part of said solid formed in step a); and
c) a liquefaction and/or sublimation of at least a part of said solid extracted in step b) so as to obtain one or more CO2-rich fluids;
characterized in that the time intervals during which said step b) is implemented are included within the time intervals during which said step a) is implemented.
The process fluid generally comes from a boiler or any plant that produces flue gases. These flue gases may have undergone various pre-treatments, notably with a view to removing NOx (oxides of nitrogen), dust, SOx (oxides of sulfur) and/or water.
Prior to separation, the process fluid is either monophasic, in gaseous or liquid form, or polyphasic. What is meant by “gaseous” form is “essentially gaseous” form, that is to say that it may notably contain dust, solid particles such as soot and/or droplets of liquid.
The process mixture contains CO2 that is to be separated from the other constituents of said fluid by cryo-condensation. These other constituents comprise one or more compounds more volatile than carbon dioxide in terms of condensation, for example oxygen 02, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2. The process fluids generally comprise predominantly nitrogen or predominantly CO or predominantly hydrogen.
In step a) the process fluid is cooled in at least one vessel. This cooling may advantageously take place at least in part by exchange of heat with CO2-rich fluids from the separation process. In addition or as an alternative, it may take place at least in part by exchange of heat with the CO2-lean gas from the separation process. These cold fluids from the separation process are heated up, while the process fluid is cooled down. This makes it possible to reduce the amount of energy required for the cooling operation.
As the cooling continues the initially gaseous CO2 is successfully solidified by raising the process fluid to a temperature below the triple point for CO2 while the partial pressure of the CO2 in the process fluid is below that of the triple point for CO2. This encourages the direct conversion of gaseous CO2 into solid CO2. For example, the total pressure of the process fluid is close to atmospheric pressure. This solidification operation is sometimes known as “cryo-condensation” or “anti-sublimation” of the CO2 or, by extension, of the process fluid.
According to one particular embodiment, all the components of the process fluid which do not solidify in step a) or which are not lumped together with the solid CO2, remain in the gaseous state. These constitute the CO2-lean gas.
Certain compounds more volatile than CO2 do not solidify and remain in the gaseous state. Together with the non-solidified CO2 these will constitute said CO2-lean gas, that is to say will constitute said gas that comprises less than 50% CO2 by volume and preferably less than 10% CO2 by volume. According to one particular embodiment, said CO2-lean gas contains less than 1% CO2 by volume. According to another particular embodiment, it contains more than 2% thereof. According to another particular embodiment, it contains more than 5% thereof. A solid comprising predominantly CO2, that is to say containing at least 90% by volume if considered in the gaseous state, preferably containing at least 95% by volume, and more preferably still containing at least 99% CO2 by volume, is formed.
This solid may comprise other compounds than CO2. Mention may, for example, be made of other compounds which might also have solidified, or alternatively of bubbles and/or drops of fluid contained within said solid lump. This explains how the solid could potentially consist of not only solid CO2. This “solid” may contain non-solid parts such as fluid inclusions (drops, bubbles, etc).
In step b), at least part of this solid is extracted from the vessel in which the cryo-condensation took place. This extraction may be achieved with no specific action, because of the shape of the vessel in which the cryo-condensation takes place, or may occur through the intervention of dedicated means. The solid extracted is possibly transported to other vessels.
The inventors have demonstrated that is particularly advantageous for energy reasons, but also for mechanical reasons, for step b) to be carried out during time intervals contained within the time intervals in which step a) is implemented.
What is meant by a time interval during which a certain step E is carried out, is the duration comprised between an instant t1 in which the step E commences and an instant t2 at which it ends, without there being any interruption in the step E between t1 and t2. A time interval [t1; t2] is said to be comprised or contained in a time interval [t3; t4] if t3 occurs before or at the same time as t1 and t4 occurs after or at the same time as t2.
Stated differently, when step b) is implemented, step a) is in the process of being performed. When the solid-extraction operations in step b) occur, the operations of cooling the process fluid and cryo-condensation also occur. This naturally assumes that the cryo-condensation and the extraction relate to different molecules.
Next, in step c), the solid is returned to temperature and pressure conditions such that it changes into a fluid, liquid and/or gaseous, state. At least part of said solid may then liquefy (melt). This then gives rise to one or more CO2-rich primary fluids. These fluids are said to be “primary” to distinguish them from treatment fluids which are said to be “secondary”. What is meant by “CO2-rich” is something “comprising predominantly CO2” within the meaning defined hereinabove.
It is moreover advantageous to carry out the first and/or the second cooling of the process fluid using one or more refrigerating cycles each comprising at least one near-isentropic expansion of a gas. These refrigerating cycles consist of several steps which cause a so-called “working” fluid to pass via several physical states characterized by given composition, temperature, pressure, etc conditions. The presence, among the steps of the cycle, of at least one near-isentropic expansion, that is to say of an expansion that causes the entropy of the expanded fluid to increase by less than 25%, preferably less than 15% and more preferably still, less than 10% makes it possible to improve the energy consumption of the separation process.
Depending on circumstances, the method according to the invention may comprise one or more of the following features:
said process fluid is essentially gaseous.
said solid containing predominantly CO2 and formed in step a) and extracted in step b) is in the form of carbon dioxide snow. It therefore has the consistency of snow, making it easier to extract.
said cryo-condensation occurs through deposition on one or more surfaces. said surfaces are the internal and/or external surfaces of tubes.
said surfaces are the external surfaces of solid particles.
said surfaces are oriented in such a way that at least part of said solid periodically falls off under the effect of gravity once a certain thickness of said solid has built up on said surfaces.
said surfaces are periodically scraped to remove at least part of said solid progressively as it forms.
said scraping is performed at least in part by one or more endless screws.
at least part of said surfaces is given a vibrating movement that helps to dislodge at least part of said solid.
at least part of said surfaces is periodically heated in order to detach at least part of said solid and cause it to fall off.
said cryo-condensation of said solid takes place on supporting solid particles that form a fluidized bed.
said supporting solid particles on which said solid is condensed in a first reactor are removed from said first reactor then regenerated in a second reactor in order to rid them of at least part of said solid which they support.
said supporting solid particles are taken from said first and second reactors by gas-solid separation in cyclones.
said supporting solid particles contain at least one metal and/or one plastic or alternatively contain predominantly CO2.
at least part of said solid is extracted in said step b) under the action of one or more endless screws.
at least part of said solid is dislodged from said surfaces or from said supporting solid particles on which said solid is condensed in step a), said dislodging being obtained under the effect of pressure waves or assisted by pressure waves.
A solid forms and adheres to the walls of the vessel in which the process mixture is cooled and cryo-condensed.
These surfaces may be of variable shapes. They may be flat or twisted. According to one particular embodiment the geometry of the vessel is tubular, that is to say that the process fluid circulates through hollow tubes and/or around hollow or solid tubes. According to another embodiment, cryo-condensation occurs at the surface of particles of varying shapes, for example beads.
The solid which forms on these surfaces can be extracted in various ways. If the surfaces or the particles on which cyro-condensation occurs are mobile, then it is not strictly necessary to dislodge the solid. If these surfaces do not leave the vessel in which cryo-condensation occurs, then it becomes necessary to detach the solid and transport it out of said vessel.
According to one particular embodiment, the surfaces in question are oriented in such a way that the solid can fall off under its own weight when a certain thickness has built up. Said surfaces may also be scraped by any mobile means of a shape suited to said surfaces. According to one particular embodiment, scraping may be performed by one or more endless screws positioned near said surfaces or in contact with said particles, at a distance such that the screw bites into or displaces the layer of solid that is to be extracted.
These surfaces, whatever they might be, may also be given a vibrational movement causing or encouraging the solid to be dislodged. The frequency and amplitude of these movements will be tailored so that detachment occurs for a certain thickness of solid.
Said surfaces may also be heated to detach all or some of the solid. According to one particular embodiment, this heating is performed by electrical tracing, that is by running heating electrical resistors through the structure of the vessel.
According to an alternative of the method, cryo-condensation takes place on particles in a fluidized bed. These particles are circulated from the regions in which cryo-condensation takes place towards regions in which the particles lose at least some of the layer of solid formed at their surface. One possible embodiment is to have one or more cryo-condensation reactors and one or more regeneration reactors between which reactors said particles circulate. These particles are generally separated from the gaseous streams by cyclones. Regeneration of the particles may or may not include re-cooling of said particles to a temperature of below the triple point temperature of CO2.
These particles may contain various materials, particularly metal and/or plastic. They may comprise solid containing predominantly CO2. In one embodiment, they grow or even appear in the cryo-condensation reactor and diminish, or even disappear, in the regeneration reactor.
The solid that has become detached may be transported away from the vessel in which cryo-condensation has occurred using one or more endless screws.
According to another embodiment, the solid may be detached from said surfaces by sending pressure waves, for example ultrasonic waves, into the cryo-condensation vessel.
All of the aforementioned means for detaching the solid deposited on said surfaces can be implemented alone or in combination.
The invention also relates to the method applied to industrial flue gases with a view to capturing CO2.
According to one particular embodiment, these flue gases come from a plant producing energy (steam, electricity) and may have undergone pretreatments.
The invention will be better understood upon reading the description and examples that follow, which are non limiting. They make reference to the attached drawings in which:
The plant illustrated in
adsorption on fixed beds, fluidized beds and/or rotary dryer, the adsorbent potentially being activated alumina, silica gel or a molecular sieve (3A, 4A, 5A, 13X, . . . );
condensation in a direct-contact or indirect-contact exchanger. the dried fluid 40 is then introduced into the exchanger 109 where the fluid is cooled down to a temperature close to, but in all events higher than, the temperature at which CO2 solidifies. This temperature can be determined by a person skilled in the art aware of the pressure and composition of the process fluid 40. This temperature is situated at around about −100° C. if the CO2 content of the process fluid is of the order of 15% by volume and for a pressure close to atmospheric pressure, the fluid 42 which has undergone a first cooling 109 is then introduced into a vessel 111 where it continues to be cooled down to the temperature that provides the desired level of CO2 capture. Cryo-condensation of at least part of the CO2 contained in the fluid 42 occurs producing, on the one hand, a CO2-lean gas 44 and, on the other hand, a solid 61 comprising predominantly CO2. The gas 44 leaves the vessel 111 at a temperature of the order of −120° C. This temperature is chosen as a function of the target level of CO2 capture. At this temperature, the CO2 content of the gas 44 is of the order of 1.5% by volume, namely a capture level of 90% starting out from a process fluid containing 15% CO2. There are various technologies that can be used for this vessel 111:
Continous solid cryo-condensation exchanger in which solid CO2 is produced in the form of carbon dioxide snow, is extracted, for example, using a screw and pressurized to introduce it into a bath of liquid CO2 121 in which a pressure higher than the triple point pressure for CO2 obtains. This pressurization can also be carried out batchwise in a system of silos. Continuous solid cryo-condensation may itself be performed in various ways:
scraped surface exchanger, the scrapers for example being in the form of screws to encourage extraction of the solid;
fluidized bed exchanger so as to carry the carbon dioxide snow along and clean out the tubes using particles for example of a density greater than that of the carbon dioxide snow;
exchanger in which solid is extracted by vibration, ultrasound, a pneumatic or thermal effect (intermittent heating so as to cause the carbon dioxide snow to fall);
accumulation on a surface with periodic “natural” fall into a tank. the fluid 46 is then heated up in the exchanger 109. As it leaves, the fluid 48 can also be used notably to regenerate the unit used for eliminating residual vapor (107) and/or for producing cold water (115) by evaporation in a direct-contact tower 115 into which a dry fluid 50 is introduced which then becomes saturated with water, vaporizing some of it; some of the cold required for the cryo-condensation carried out in the vessel 111 is supplied by one or more cold sources (75). Likewise, some of the cold required for the first cooling 109 is supplied by one or more cold sources (76); the solid 62 comprising predominantly CO2 is transferred to a bath 121 of liquid CO2; this bath 121 needs to be heated in order to remain liquid, to compensate for the addition of cold from the solid 62 (latent heat of fusion and sensible heat). This can be done in various ways:
by exchange of heat with a hotter fluid 72,
by direct exchange, for example by tapping a fluid 80 from the bath 121, heating it in
the exchanger 109, and reinjecting it back into the bath 121. liquid 64 comprising predominantly CO2 is tapped from the bath 121. this liquid is split into three streams. In the example, the first is obtained by an expansion 65 to 5.5 bar absolute producing a diphasic, gas-liquid, fluid 66. The second, 68, is obtained by compression 67, for example to 10 bar. The third, 70, is compressed for example to 55 bar. The 5.5 bar level provides cold at a temperature close to the triple point temperature for CO2. The 10 bar level allows the transfer of the latent heat of vaporization of the fluid 68 at around −40° C. Finally, at 55 bar, the fluid 70 does not vaporize during the exchange 109. There is efficient use to be made of the cold energy contained in the fluid 64 during the exchange 109 while at the same time limiting the amount of energy required to produce a purified and compressed stream 5 of CO2; after the exchange 109, the primary fluids 66, 68, 70 are compressed to a pressure level higher than the critical pressure for CO2 using the compressors 131, 132, 133.
A primary airflow 15 passes through the unit 3 in which the coal 15 is pulverized and carried along toward the burners of the boiler 1. A secondary airflow 16 is applied directly to the burners in order to provide additional oxygen needed for near-complete combustion of the coal. Feed water 17 is sent to the boiler 1 to produce steam 18 which is expanded in a turbine 8.
The flue gases 19 resulting from the combustion, comprising nitrogen, CO2, water vapor and other impurities, undergo various treatments to remove some of said impurities. The unit 4 removes the NOx for example by catalysis in the presence of ammonia. The unit 5 removes dust, for example using an electrostatic filter, and the unit 6 is a desulfurization system for removing the SO2 and/or SO3. The units 4 and 6 may be superfluous depending on the composition of the product required. The purified flow 24 from the unit 6 (or 5 if 6 is not present) is then sent to a low-temperature cryo-condensation purification unit 7 to produce a relatively pure flow 26 of CO2 and a nitrogen-enriched residual flow 25. This unit 7 is also known as a CO2 capture unit and implements the method that forms the subject of the invention, as illustrated, for example in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0858863 | Dec 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR09/52506 | 12/14/2009 | WO | 00 | 8/24/2011 |
